In a long-distance transmission system having a transmission speed of 40 G bits/second or more, generally, a modulation system having a low symbol rate is used to improve tolerance to chromatic dispersion or polarization dispersion. For example, it is desirable that a multi-level phase modulation system be used to reduce a symbol rate.
However, in a transmitter and a receiver using the multi-value phase modulation system, there are a lot of locations to be controlled in the transmitter, which thus leads to an increase in the complication of the transmitter and in which performing adjustments (for example, bias control of Lithium Niobate (LN) modulator, etc.) of the transmitter is time consuming, both results being undesirable.
In addition, it is necessary that the adjustment of the transmitter be performed under a condition in which an optical signal is output, so that the optical signal output during the adjustment of the transmitter is input into a receiver at the opposite side of the transmitter through a transmission path.
The receiver at the opposite side of the transmitter is not allowed to determine whether the input optical signal is a normal optical signal or an optical signal at the adjustment stage. In addition, the receiver proceeds to a regular operation at a time when transmission of a main signal has been detected. Thus, when the receiver receives an optical signal at the adjustment stage and detects that a main signal has been transmitted, the receiver proceeds to a regular operation, undesirably.
FIG. 1 illustrates an example of the output level of a signal output from a transmitter. Referring to FIG. 1, in the transmitter, the output level of a signal varies dynamically, from the point in time at which the transmitter outputs an optical signal, to an adjustment stage, so that a signal having a stable output level is not allowed to be output. Next, after completion of the state in which the transmitter has been adjusted, a signal whose output is stable is allowed to be output. As illustrated in FIG. 1, fluctuation of an optical level may occur when adjustment of the transmitter is performed. However, the state illustrated in FIG. 1 is a mere example, and transmission waveform may lose shape even when an optical level is constant.
As illustrated in FIG. 1, signal quality in an optical signal at the adjustment stage is worse than signal quality in an optical signal at an adjustment completion stage (regular operation stage), so that communication quality deteriorates and desired communication characteristics are not allowed to be satisfied when the receiver proceeds to a regular operation in a state in which an optical signal at the adjustment stage is received. Thus, the development of a technology is desired in which the state of the transmitter is recognized by a receiver, thereby preventing the receiver from proceeding to a regular operation in a state in which an optical signal at the adjustment stage is received.
In addition, for example, there is JP 62-51321A as a related technology document. In JP 62-51321A, a technology is discussed in which optical input level information monitored by an optical/electric (O/E) conversion circuit of a receiving section is transmitted to an apparatus at the opposite side of the receiver as overhead information, and the optical output level of an electric/optical (E/O) conversion circuit in the transmission section of the apparatus at the opposite side of the receiving section is controlled based on the information.
According to the technology described in JP 62-51321A, the load of a light emitting element is properly adjusted depending on the transmission path distance by controlling the optical output level of the E/O conversion circuit, thereby reducing the load in a case of a short distance. In addition, in the case of a short distance, an undesirable excessive optical input to a light receiving element is allowed to be prevented.
In JP 2008-135821A, another technology is discussed in which comfort noise (CN) characteristics are detected from a received signal at a receiving station, the CN characteristics are transmitted to the transmitting station, an input level to an E/O converter is adjusted at the transmitting station based on the CN characteristics, the input level adjustment information is transmitted to the receiving station, and gain is adjusted at the receiving station side based on the input level adjustment information.
According to the technology described in JP 2008-135821A, transmission and reception are allowed to be performed in an optimal state by performing transmission and receiving various pieces of information between the receiving station and the transmitting station regardless of the state of an optical transmission section.
In JP 62-51321A and JP 2008-135821A, technologies are discussed in which information is transmitted between the transmitter and the receiver, and various control is performed based on the information, however, a technology is not discussed in which the transmitter is allowed to be recognized by the receiver.